The invention relates to a transmission and receiving device for contactless data transmission between the device and at least one transponder, including a drive stage for generating a drive signal and an antenna resonant circuit which includes a coil and a capacitor, which can receive the drive signal and in which an RF signal occurs which corresponds to the drive signal and can be modulated by load modulation in dependence on data to be transmitted to the transmission and receiving device by a transponder, and also including an evaluation unit which can receive the RF signal for the purpose of evaluation and is arranged to determine data contained in the RF signal due to the load modulation, a measuring unit for determining measurement values of an electric variable which occurs in the antenna resonant circuit, and a control unit for generating control information in dependence on the measurement values determined.
A transmission and receiving device of the kind set forth in the preamble is known, for example from the document EP 0 625 832 A1. The known transmission and receiving device is formed as a read apparatus intended for contactless transmission of energy and data between the apparatus and at least one detection plate which constitutes a transponder.
The known transmission and receiving device includes an antenna resonant circuit which consists of a coil and at least one capacitor which determine the self-resonant frequency of the antenna resonant circuit. The self-resonant frequency of the antenna resonant circuit may deviate from a given nominal self-resonant frequency due to tolerances of components of the transmission and receiving device or due to temperature influences. In the known transmission and receiving device the self-resonant frequency of the antenna resonant circuit of the transmission and receiving device is tuned as well as possible to the given nominal self-resonant frequency by means of an automatic tuning operation which is carried out, for example at regular intervals, resulting in a comparatively good inductive coupling between the antenna resonant circuit of the transmission and receiving device and a transponder antenna resonant circuit of a transponder co-operating with the transmission and receiving device.
To this end, a tuning unit for the antenna resonant circuit, being capable of varying the self-resonant frequency of the antenna resonant circuit, is connected parallel to a main capacitor of the antenna resonant circuit. The tuning unit includes a plurality of parallel-connected additional capacitors which can be connected parallel to the main capacitor of the antenna resonant circuit by control signals which can be applied to the tuning unit and constitute a respective control information, thus enabling variation of the overall capacitance of the respective parallel-connected capacitors of the antenna resonant circuit by the respective control information.
The known transmission and receiving device also includes a measuring unit which measures the voltage across the main capacitor of the antenna resonant circuit and supplies information concerning the voltage across the main capacitor to a control unit in the form of a microcomputer.
The control unit supplies the tuning unit with control information, on the basis of which the overall capacitance of the respective parallel-connected capacitors is varied, so that the self-resonant frequency of the antenna resonant circuit, and hence the voltage across the main capacitor of the antenna resonant circuit, changes. The control unit is informed about the variation of the self-resonant frequency of the antenna resonant circuit via the information concerning the voltage measured across the main capacitor of the antenna resonant circuit by the measuring unit.
During the automatic tuning operation, the control unit of the known transmission and receiving device supplies the tuning unit with control information in order to change the overall capacitance of the respective parallel-connected capacitors of the antenna resonant circuit until the measuring unit measures a maximum voltage across the main capacitor of the antenna resonant circuit at a given nominal self-resonant frequency, i.e. the operating frequency of the transmission and receiving device and the transponder.
A transponder which is intended to co-operate with the known transmission and receiving device is provided with a transponder antenna resonant circuit.
This transponder antenna resonant circuit is inductively coupled to the antenna resonant circuit of the transmission and receiving device in a communication mode, so that a transponder RF signal which corresponds to the RF signal which corresponds in the antenna resonant circuit occurs in the transponder antenna resonant circuit, which transponder RF signal can be load modulated in dependence on digital data that can be read from a memory of the transponder. In the case of a suitable relationship of the self-resonant frequencies of the antenna resonant circuits, the load modulated transponder RF signal causes an amplitude modulated RF signal in the antenna resonant circuit of the transmission and receiving device, due to the inductive coupling of the antenna resonant circuits, the amplitude modulation of the RF signal then containing the digital data transmitted from the transponder to the transmission and receiving device.
In the known transmission and receiving device the data contained in the amplitude modulated RF signal can be extracted in known manner by amplitude demodulation of the amplitude-modulated RF signal by means of the evaluation unit.
The described automatic tuning operation keeps the level of the RF signal at an as high as possible value by the tuning of the antenna resonant circuit at the side of the transmission and receiving device, so that amplitude demodulation of the RF signal so as to extract the digital data is facilitated.
However, it has been found that in the known transmission and receiving device in many cases no digital data can be extracted by amplitude demodulation of the modulated RF signal, despite a high level of the modulated RF signal. This is because, in the case of an unfavorable relation of the self-resonant frequencies of the antenna resonant circuits of the transmission and receiving device and a transponder, the digital data occurs in the modulated RF signal not in the form of an amplitude modulation but in the form of a phase modulation. In these cases, because of the change of modulation no digital data can be extracted by amplitude demodulation in the transmission and receiving device so that the communication between a transponder and the transmission and receiving device is disturbed or interrupted which is, of course, undesirable.